I understand that Rivera routinely gets calls that are 2-3 inches off the plate, but according to pitchfx, that pitch to McDonald was 2-3 inches off the plate. His overall results weren't that bad considering the error, but his control is off and he's coming off an appearance in which he walked in a run and allowed a grand slam. That's not typical Mo and he's coming off an injury. I think there's a very good chance that he's no 100%, he's inhuman when he's healthy.
EDIT: And overall he didn't get the benefit of two "Mo" strikes (2-3 inches off the plate), but he also got the benefit of two called strikes that were more than 2-3 inches off the plate as you can see from this graph. The two called strikes you can see to the left of the box are 3.5 inches off the plate so it's hard to blame this one on the umps.
http://www.brooksbaseball.net/pfxVB/cache/location.php-pitchSel=121250&game=gid_2010_05_18_bosmlb_nyamlb_1&batterX=&innings=yyyyyyyyy&sp_type=1&s_type=.gif